Field and laboratory investigation on geotechnical properties of sewage sludge disposed in a pit at Changan landfill, Chengdu, China

Abstract

In order to find a proper treatment method for a sewage sludge disposal pit, field and laboratory investigation were conducted on the geotechnical properties of the sludge at the Changan landfill in Chengdu, China. The field investigation included borehole sampling, cone penetration tests and vane shear tests. The laboratory study involved the measurements of volume–mass relation, hydraulic conductivity, compressibility, coefficient of consolidation and shear strength for the borehole samples taken from the field. The field investigation revealed that the sludge in the disposal pit could be subdivided into three layers, i.e., top crust layer, middle fluid-like layer and deep soft plastic layer. The vane shear strength for the sludge was less than 1kPa at the fluid-like layer, and 1 and 3kPa at the deep soft plastic layer, being significantly lower than that of soft clay. Laboratory test results demonstrated that the sludge was high in organic content (18.5% to 51.8%) and water content (from 210% to 790%), and both of them exhibited a decreasing trend with depth. The variation in the organic content indicated a difference in the degree of biodegradation for the sludge at locations. The consolidation property and shear strength of the sludge exhibited a non-linear relationship to the applied consolidation pressure. The coefficient of consolidation of sewage sludge, decreasing with increasing consolidation pressure, was in an order of 10−5cm2/s to 10−6cm2/s, being 1–2 order lower than that of soft clay. The friction angle for the sludge increased from 10° for the pressure range from 0 to 100kPa to 15° for the pressure range from 100 to 400kPa.